A laser beam is constituted by a high energy luminous wave which originates in a cavity comprising several mirrors between which it performs a number of back and forth movements before being extracted to be directed toward a work station. The energy distribution of a laser beam cut in a plane perpendicular to its axis depends on the geometry of the cavity in which it was generated.
To simplify things, the gaussian beam, which is at present the most widespread, will serve solely as an example, the invention being nevertheless applicable to other shapes of beam without limitation of its performance.
In a gaussian beam, a large part of the energy is concentrated at the center of the spot, and this energy decreases in correspondence with the distance from the axis of the beam along a Gauss curve. In practice, however, the beam is never perfectly gaussian and always has several peaks due to problems of optical aberration, dirt, "wear" and mirror adjustment, etc. . . . These peaks can often be minimalized by precise adjustment of the alignment of the mirrors.
The quality of the beam depends generally on the energy distribution, which conditions the performances during machining operations, such as cutting, welding, thermal treatments, or others, and which must be perfectly controlled. This control of the distribution of the energy within the beam is at present very difficult to effect by simple and inexpensive means, contrary to the adjustment of the various mirrors of the cavity.
At present, such control is effected either by firing into polymethylmethacrylate, or by means of beam analyzers.
Firing into polymethylmethacrylate gives a print of the beam which is, however, unreliable because of alteration of the print by polymethylmethacrylate vapors, thereby permitting only roughly qualitative indications.
Known beam analyzers, which rely on silicon detector technology, permit visualizing on the screen of an oscilloscope the energy distribution within a laser beam, by withdrawing a portion of the incident energy.
But these devices provide results which are hardly reproducible because of the difficulties of adjustment, of the moving members, of the intermediate structure between the beam and the detectors and the small portion of the beam useful for measurement. Moreover, certain of these devices show the profile of energy distribution in the beam only on two axes, without giving the overall shape of this distribution.
There also exist other analysis devices, used in laboratory settings, which permit obtaining good results but which however are not usable industrially because of their excessive complexity.
There is also known, from FR-A-2 619 475, an analysis apparatus of the energy distribution within a power laser beam, constituted essentially by a thermoelectric receiver directly exposed to the laser beam and delivering electrical signals to an intermediate demultiplexer and amplifier, and by a computer or a microcomputer for treating measurements and visualizing in real time the energy distribution in the beam, connected to the intermediate device and piloting this latter, characterized in that it functions in a continuous mode and in an impulse mode, and in that the thermoelectric receiver is constituted by a sealed body, whose surface turned toward the source of the laser beam is metallic and constitutes, in cooperation with metallic wires, the support of the thermocouples disposed on coaxial circles or according to a square or rectangular locus, or also according to a cross or a star, the wires passing in sealed relation through the surface of the body opposite the surface turned toward the source of the laser beam, and the body being provided moreover with two circulation openings for a cooling fluid.
The thermocouples are preferably iron-constantan couples, the surface of the receiver body being comprised by a sheet of pure iron, whose surface condition is uniform, and metal wires being of constantan; the soldering between each wire of the thermocouple and the sheet forming the surface is effected either by a capacitative discharge arc of a row of condensers, or by means of a pulsed laser, by soldering.
The arrangement of the thermocouples is a function of the precision of analysis or required resolution, of the diameter of the beam in the analysis plane, and of the number of channels of the intermediate demultiplexer device to which said thermocouples are connected, and the spacing between each of the thermocouples depends on the diameter of the wires and of the soldering technique that is used.
The surface of the sealed body turned toward the source of the laser beam is cooled by forced circulation of a cooling liquid, generally water, whose flow rate is adjusted so as to permit the establishment of the thermal regime.
The analysis device according to FR-A-2 619 475 is pivotally mounted on an axle parallel to the axis of the laser beam, either externally in front of the output port of the laser cavity, or internally in front of said port within the laser cavity. Thus, to effect a measurement, it suffices to intercept the laser beam by interpositioning the device, this latter being pivoted out of the beam after measurement.
The receiver can also be mounted perpendicularly to the axis of the laser beam and can coact, for measurement, with a removable mask constituted by a mirror inclined at 45.degree.relative to the axes of the laser beam and the receiver, and adapted to be disposed in the path of the laser beam by means of a jack or cam at the time of measurement. Thus, the device permits controlling the beam which is reflected by the mirror onto the receiver before utilization at the work station. In such a case, the control of the beam is effected during dead time, using the receiver as a light trap.
The receiver can also be mounted obliquely relative to the laser beam and coact with a removable mask inclined correspondingly, or can replace this latter, and can reflect the energy not absorbed by the light trap.
In the case of a control of the beam outside the laser cavity, the said device can permit effectuating diagnosis of the condition of the optics by effecting controls of the laser beam before and after each of the optical elements. The device is also utilizable for verifying the directional stability of the mode and size of the beam.
The computer or microcomputer for treating the measured values and for visualizing in real time the distribution of the energy in the beam is connected to the intermediate device by an analog-digital converter, and controls said device by means of a parallel interface. This computer or microcomputer, which comprises in known manner a central unit, a mass memory and a display screen, is connected also to a printer.
This device permits indeed control of the position of the mirrors of the laser cavity by rapid interpretation of the recorded measurements, and thus permits effecting adjustment substantially in real time of said mirrors of the laser cavity, but requires nevertheless a repetition of the measurements after each mirror by displacement and insertion of the receiver in the beam behind each mirror, such that the analysis of the energy distribution remains relatively long in spite of everything.
Moreover, this receiver does not permit simultaneous verification of the alignment of the beam.